Method of forming ohmic contact to a semiconductor body

ABSTRACT

A process for forming an ohmic contact on the back surface of a semiconductor body includes depositing a donor layer on the back surface of the semiconductor body followed by a sintering step to form a shallow intermetallic region capable of forming a low resistance contact with a contact metal.

RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 11/169,820,filed Jun. 29, 2005, entitled “METHOD OF FORMING OHMIC CONTACT TO ASEMICONDUCTOR BODY” which is based on and claims priority to U.S.Provisional Application No. 60/584,073, filed Jun. 29, 2004, by GiovanniRichieri, entitled “IMPROVED OHMIC CONTACT TO SILICON CARBIDE WAFERS ORTO N⁻SILICON AND PROCESS THEREFOR,” the contents of which are hereinincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to semiconductor devices, and morespecifically, to ohmic contact structures for float zone silicon wafersand for silicon carbide wafers and to methods for fabricating suchcontact structures.

2. Description of Related Art

It is well known to form a semiconductor device using a semiconductordie that includes a N⁻ epitaxial silicon layer formed atop a N⁺ floatzone silicon substrate, for example, the device being formed in thesurface of the epitaxial layer and electrode contact metals being formedon the upper and back surfaces of the die. For example, a fast recoverydiode (FRED) generally includes a plurality of PN junctions formed inthe upper portion of the epitaxial layer and spaced by Schottky regions.A common top electrode is shared by the PN junctions and Schottkyregions to form a device that includes PN and Schottky diodes. A backelectrode is formed along the back surface of the N⁺ float zone siliconsubstrate. The need to grow the epitaxial layer on a semiconductor die,however, makes the die expensive, whether used for a FRED or any otherdevice.

Accordingly, it would be desirable to form a device directly in/on thesurface of a float zone silicon die that does not have the epitaxiallayer. Once forming device components in/on the upper surface of thedie, a back surface contact metal must thereafter be formed. In aconventional device, the presence of a high conductivity region at theback surface of the die (e.g., N⁺ or P⁺ region) lowers the contactresistance between the contact metal and the silicon body, therebyresulting in a good ohmic contact. However, because a float zone die isusually lightly doped (e.g., N⁻ or P⁻) it does not form a good ohmiccontact with a contact metal.

Notably, an implant can be performed along the back surface of the dieto improve its capability of forming a good ohmic contact. Such a step,however, requires additional handling, which may be undesirable when anultrathin device is desired. In addition, an implant may not be feasiblewhen an ultrathin device is desired.

Similarly, it is known to form a device in the upper surface of asemiconductor die made of silicon carbide. Here again, a back sidecontact metal needs to be formed such that the contact metal makes goodohmic contact to the silicon carbide. However, the formation of suchohmic contacts has also been quite difficult to form, as described inU.S. Pat. No. 5,980,285. For example, it has been difficult tosufficiently dope the silicon carbide surface to form an N⁺ region forcontact by a contact metal.

SUMMARY OF THE INVENTION

Accordingly, it is desirable to form an ohmic contact between a contactmetal and a back surface of a float zone silicon die or silicon carbidedie, thereby overcoming the above and other disadvantages of the priorart. According to a preferred embodiment of the invention, components ofa semiconductor device are first formed in/on the upper surface of a N⁻float zone silicon body using any well known process. For example, aplurality of interleaved spaced PiN diodes and Schottky contacts for afast recovery diode may be formed into the upper surface of the siliconbody, together with a termination. Thereafter, a back surface of thesilicon body may be ground as by grinding and/or etching to obtain, forexample, an ultrathin device. For example, the silicon body may bethinned to about 60 to 75 μm.

Thereafter, a bottom contact is made to the back surface of the N⁻ body.Specifically, according to an embodiment of the invention, a donor layerthat includes atoms capable of forming an intermetallic composition withlow contact resistivity is first deposited on the back surface of thesilicon body by sputtering or evaporation and is thereafter sintered ata temperature less than about 500° C. As a result, a shallowintermetallic region of N⁺⁺ conductivity is formed within the backsurface of the silicon body. For example, the N⁺⁺ intermetallic regionmay have a depth into the back surface of the silicon body of about 0.5to 1.6 μm. Note that the donor layer may not completely diffuse into thebody, however, any un-diffused material is not removed. According to afurther aspect of the invention, a thin titanium layer or atitanium-tungsten alloy layer may be placed between the donor layer andsilicon body surface to enable the use of a high sinter temperature. Thetitanium or titanium-tungsten layer is used as promotion adhesion forthe final metal.

According to the present invention, the donor layer may include an alloyof gold, such as gold with arsenic (AuAs), gold with antimony (AuSb), orgold with tin (AuSn).

Once the intermetallic region is formed, a contact metal, such as aconventional trimetal like chromium/nickel/silver, for example, isdeposited over the intermetallic region, thereby forming an ohmiccontact. Preferably, the depositing and heating of the donor layer andthe depositing of the contact metal are performed without additionalhandling of the device.

According to a preferred embodiment of the invention, a similar contactprocess is employed for silicon carbide. Specifically, once formingcomponents of a device in an upper surface of a silicon carbide body, adonor layer is deposited on the back surface thereof, preferably throughsputtering. The donor layer is then annealed, in situ, at 350° C. to450° C. for a few minutes creating an intermetallic region of N⁺conductivity within the back surface. According to the presentinvention, the donor layer may include an alloy of gold, such as goldwith antimony (SbAu), that is sputtered to 3000 Å thick or less. Oncethe intermetallic region is formed, a back side contact metal, such asnickel (Ni) or an alloy of titanium, nickel, and silver (TiNiAg), forexample, is then applied to the intermetallic region to form an ohmiccontact.

According to a further embodiment of the invention, the above describedprocess for forming an ohmic contact to a silicon carbide substratesurface can also be used to form an ohmic contact to a gallium nitride(GaN) substrate surface.

Other features and advantages of the present invention will becomeapparent from the following description of the invention, which refersto the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross sectional view of a semiconductor device having anohmic contact structure on a back side of a N⁻ float zone silicon bodyaccording to an embodiment of the present invention.

FIGS. 2-13 graphically illustrate a process for fabricating thesemiconductor device of FIG. 1.

FIGS. 14 and 15 graphically illustrate a process according to anembodiment of the present invention for forming the ohmic contactstructure of FIG. 1.

FIG. 16 is a plot of doping concentrations versus depth for thesemiconductor device of FIG. 1.

DESCRIPTION OF THE EMBODIMENTS

Referring to FIG. 1, there is shown in cross section a small portion ofa semiconductor device 100 (here a fast recovery diode) with a backsurface ohmic contact structure according to a preferred embodiment ofthe present invention (note that FIG. 1 is not drawn to scale).Specifically, device 100 includes float zone silicon body 102 of N⁻conductivity. Within the upper surface of body 102, along active area150, are a plurality of spaced diffusions 152 of P⁺ conductivity. Acontact metal 154 (e.g., aluminum) contacts the surface of diffusions152, thereby defining a plurality of PiN diodes 156, and furthercontacts the surface of body 102 between diffusions 152, therebydefining a plurality of Schottky contacts 158. As illustrated, contactmetal 154 forms an anode electrode.

Termination region 160 resides at the periphery of active area 150.Termination region 160 includes a plurality of spaced field oxide rings162 a-d formed along the surface of body 102. Within the surface of body102 are a plurality of guard rings 164 of P⁺ conductivity, each guardring being situated between two adjacent field oxide rings 162. Atopeach guard ring 164 and between adjacent field oxide rings 162 is oxidelayer 166, each layer having a window/gap 165 formed therein thatextends to the surface of the underlying guard ring. Over each fieldoxide ring is a corresponding stepped field plate 168 a-d (e.g.,aluminum), with field plate 168 a contacting contact metal 154 and withfield plates 168 b-d extending through a window 165 and contacting aguard ring 164.

Laterally surrounding the die edge within separation region/cuttingstreet 170 is N⁺ diffusion 172. Underlying field plate 168 d, along theouter periphery of the termination region, and overlying diffusion 172is oxide layer 176.

Overlying field plates 168 a-d, a portion of contact metal 154, anddiffusion 172 is amorphous silicon layer 180. Platinum atoms may bediffused into body 102 to act as a lifetime killer in the usual manner.

Along the back surface of body 102 is contact metal 190 (e.g., aconventional trimetal) forming a cathode electrode. According to anembodiment of the present invention, beneath contact metal 190 andwithin the back surface of body 102 is a shallow intermetallic region180 of N⁺⁺ conductivity. Through intermetallic region 180, an ohmiccontact is formed between contact metal 190 and body 102.

Referring now to FIGS. 2-15 (note that the Figures are not drawn toscale), there is illustrated a process for fabricating fast recoverydiode 100 and in particular, a process for forming an ohmic contactbetween a back surface contact metal and a N⁻ float zone silicon bodyaccording to a preferred embodiment of the invention. Beginning withFIG. 2, there is illustrated a cross-section of a small portion of aninitial body 102 of starting float zone silicon having a crystalorientation of <111>, a non-critical thickness of about 375 μm, and N⁻conductivity. In this example, silicon body 102 has resistivity of about20 to 30 ohm cm. However, this resistivity is not required and differentresistivities can be used, depending on the desired voltage rating ofthe final device being made.

Once performing an initial clean, field oxide layers 104 and 106 areformed on the upper and back surfaces, respectively, of body 102, asillustrated in FIG. 3. Field oxide layer 104 may have a thickness, forexample, of about 14,000±500 Å.

Referring to FIG. 4, a first mask is next formed on the surface of oxidelayer 104 and the oxide layer is thereafter etched to the surface ofbody 102, thereby forming a plurality of windows 108 in active area 150and a plurality of windows 110 a-d in termination region 160. Asillustrated, this etching step also forms field oxide rings 162 a-d intermination region 160.

After a suitable clean, a boron implant is next carried out to deposit Ptype boron through windows 108 and 110 a-c and into the upper surface ofbody 102. As a result, implant regions 112 are formed in active area 150and implant regions 114 are formed in termination region 160, asillustrated in FIG. 5.

Referring to FIG. 6, after another clean, a boron oxidation step iscarried out, diffusing the boron implant regions 112 and 114 and growingoxide layers 116 and 166, respectively, atop each boron diffusion. As aresult, diffusions 152 of P⁺ conductivity are formed in active area 150and guard rings 164 of P⁺ conductivity are formed in termination region160. This oxidation step also grows oxide layer 176 along the surface ofbody 102 within window 110 d.

Thereafter, a second mask step is carried out to etch away oxide layer104 within separation region/street area 170, as illustrated in FIG. 7.The resulting structure is then cleaned and a PO Cl₃ deposition iscarried out to deposit PO Cl₃ within body 102 along separationregion/street area 170. The resulting structure is then deglassed andcleaned. The PO Cl₃ is then diffused in a further oxidation step,forming N⁺ diffusion 172 and growing oxide layer 176 atop thisdiffusion, as illustrated in FIG. 8.

Referring to FIG. 9, a third mask step is next carried out in which,after a hard oven bake, each oxide layer 166 is etched to formwindows/gaps 165. In addition, field oxide layer 104 and oxide layers116 are stripped from atop active area 150, and field oxide layer 106 isstripped from the back surface. Thereafter, platinum may be applied tothe back side of body 102 and diffused into the body to act as alife-time killer in the usual manner.

Referring to FIG. 10, the structure of FIG. 9 is next cleaned and asuitable metal layer 118, such as aluminum, is sputtered atop thestructure to a thickness, for example, of 3 μm. Within active area 150,metal layer 118 contacts P⁺ diffusions 152, thereby defining a pluralityof PiN diodes 156. Metal layer 118 also contacts the surface of N⁻ body102 between diffusions 152, thereby defining a plurality of Schottkycontacts 158. Within termination region 160, metal layer 118 extendsthrough windows 165 and contacts guard rings 164.

Referring to FIG. 11, a fourth mask step is next carried out in which,after a hard oven bake, metal layer 118 is etched. As a result, contactmetal 154 (i.e., an anode contact) is formed along active area 150 and aseries of stepped field plates 168 a-d are formed over field oxide rings162 a-d, respectively. As indicated, field plate 168 a contacts contactmetal 154 and field plates 168 b-d extend through windows 165 andcontact guard rings 164.

Thereafter, as shown in FIG. 12, an amorphous silicon layer 180, about1800 Å thick for example, is deposited atop the structure of FIG. 11.Amorphous silicon layer 180 is then etched to overlie field plates 168a-d, a portion of contact metal 154, and N⁺ diffusion 172, asillustrated in FIG. 13.

Next, the resulting structure as shown in FIG. 13 is suitably preparedand using a back grind and/or etch along the back side 182 of thestructure, the silicon body is ground/thinned from about 375 μm to about60 to 75 cm. Thereafter, back side 182 is subjected to a premetal clean.

Turning now to contact metal 190, as indicated, because body 102 has N⁻conductivity, an ohmic contact is not easily formed between the contactmetal and body 102. As such, according to an embodiment of theinvention, a thin donor layer 184 that includes atoms capable of formingan intermetallic composition with low contact resistivity is firstdeposited on surface 182 by sputtering or evaporation, as illustrated inFIG. 14, and is thereafter sintered at a temperature less than about500° C. As a result, a shallow intermetallic region 180 of N⁺⁺conductivity is formed within the back surface of body 102, asillustrated in FIG. 15. Note that the donor layer may not completelydiffuse into body 102, however, any un-diffused material is not removed.According to a further aspect of the invention, a thin titanium layer ortitanium-tungsten alloy layer (not shown), for example, can also beplaced between surface 182 and donor layer 184 (in FIG. 14) to enablethe use of a high sinter temperature. The titanium or titanium-tungstenlayer is used as promotion adhesion for the final metal.

According to the present invention, donor layer 184 may include an alloyof gold, such as gold with arsenic (AuAs), gold with antimony (AuSb), orgold with tin (AuSn). In particular, donor layer 184 may be an alloy ofgold with 0.1 to 0.5% arsenic, an alloy of gold with 10 to 30% antimony,or an alloy of gold with 10 to 30% tin.

Once intermetallic region 180 is formed, contact metal 190 is depositedover this surface thereby forming an ohmic contact, the resultingstructure being shown in FIG. 1. Contact metal 190 may be a conventionalthick back contact layer, such as a conventional trimetal like chromium(at 1000 Å), nickel (at 4000 Å), and silver (at 6000 Å), for example.Preferably, the depositing and heating of donor layer 184 and thedepositing of contact metal 190 are performed without additionalhandling of the device.

Referring to FIG. 16, there is shown a plot of doping concentrationversus depth into the top surface of example diode 100. As seen, the N⁺⁺intermetallic back surface 180 has a depth of about 0.5 to 1.6 μm.Accordingly, intermetallic region 180 is shallow, thereby allowing forthe formation of thin devices. For example, device 100 has a totalthickness of 75 μm or less. In addition, and as indicated above,intermetallic region 182 can be formed at low temperatures, less thanabout 500° C. The low temperature nature of the process is preferred asit will not adversely effect features already formed.

Notably, the above process for forming an ohmic contact to the backsurface of a N⁻ float zone silicon body is not limited to fast recoverydiodes and is suitable to any process for forming any planar or trenchtype device within a lightly doped float zone silicon body.

According to a preferred embodiment of the invention, the abovedescribed process for forming an ohmic contact between a back sidecontact metal and a float zone silicon body can be used to apply acontact metal to a silicon carbide substrate surface. Specifically, onceforming components of a device in an upper surface of a silicon carbidebody, a donor layer is deposited on the back surface of the body,preferably through sputtering. The donor layer is then annealed, insitu, at 350° C. to 450° C. for a few minutes creating an intermetallicregion of N⁺ conductivity within the back surface of the body. Accordingto the present invention, the donor layer may include an alloy of gold,such as gold with antimony (SbAu) and in particular, gold with 30%antimony, that is sputtered to 3000 Å thick or less. Once theintermetallic region is formed, a back side contact metal, such asnickel (Ni) or an alloy of titanium, nickel, and silver (TiNiAg), isthen applied to the intermetallic region to form an ohmic contact.

According to a further embodiment of the invention, the above describedprocess for forming an ohmic contact to silicon carbide can also be usedto form an ohmic contact to a gallium nitride (GaN) substrate surface.Gallium nitride is one example of a group III-V semiconductor.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein.

1. A semiconductor device, comprising: a float zone silicon substratehaving N type conductivity, said substrate having a top surface and aback surface; a plurality of PiN diodes and Schottky contacts formedwithin said top surface of said substrate along an active region; anintermetallic region of low contact resistivity in said back surface ofsaid substrate; and a contact metal in contact with said intermetallicregion forming an ohmic contact.
 2. The semiconductor device of claim 1,wherein said device has a thickness of 75 cm or less.
 3. Thesemiconductor device of claim 1, wherein said intermetallic region has adepth of approximately 0.5 to 1.6 μm.
 4. The semiconductor device ofclaim 1, wherein said intermetallic region includes either AuAs atoms,AuSb atoms, or AuSn atoms
 5. The semiconductor device of claim 4,further comprising: a plurality of spaced field oxide rings atop saidupper surface of said substrate along a termination region; a pluralityof guard rings of P type conductivity within said upper surface of saidsubstrate, each guard ring being between adjacent field oxide rings; agapped oxide layer atop said upper surface of said substrate and betweenadjacent field oxide rings, each gapped oxide layer exposing anunderlying guard ring; and separate stepped field plates over each ofsaid field oxide rings, wherein each field plate contacts either ananode contact metal or one of said guard rings through one of saidgapped oxide layers.